With the advent of sophisticated copying machines, especially those employing a xerographic color copying process, there has arisen a need for a device which can be incorporated in currency and which can defeat counterfeiting of the currency by such copying machines while at the same time permitting the man in the street easily to authenticate the validity of a piece of currency without the use of sophisticated and expensive authenticating apparatus.
Current anti-counterfeiting techniques include the use of photochromics, holographics, diffraction gratings and other optically variable devices which create in the currency an easily recognizable characteristic which cannot be readily duplicated, copied or counterfeited by xerographic, photographic and other reprographic processes. More specifically, Australian Patent No. 488,652 discloses incorporating in a bank note various optically variable devices, including a thin film liquid crystal material. Furthermore, devices employing a liquid crystal device driven by a photovoltaic element, such as a solar cell or an amorphous silicon material, are disclosed in U.S. Pat. Nos. 4,095,217 and 4,281,208. The following U.S. patents are also incorporated by reference herein and provide background information on the technology involved in the present invention U.S. Pat. Nos.: 4,251,136; 4,133,697; 4,149,665; 4,228,574; 3,758,305; 3,635,981; 4,308,163; and 4,308,164.